Wimbish village, Essex - Thomas Armstrong (Concrete Blocks Ltd)

Norfolk-based Parsons & Whittley architects employed Passivhaus principles in the design of what is set to be the UK’s first rural affordable housing scheme to gain Passivhaus certification in Wimbish, Essex. Their design for the development, which is being built by Bramall Construction and assessed against Passivhaus standards by Inbuilt, is simple without unnecessary steps and staggers that would add to the heat loss area and complicate the design and construction. A number of options were evaluated before adopting solid aircrete external wall construction, using 190mm blocks supplied by Thomas Armstrong, wrapped externally in 285mm of rendered insulation. Extruded polystyrene insulation has been used under a reinforced concrete ground floor slab, with conventional standard trussed rafter roofs supporting 500mm Crown Loft Roll. The construction details have purposely been kept simple, ensuring they can easily be replicated.

Key to effective insulation was the use of a new blue extruded polystyrene foam insulation, which runs below the concrete floor slabs creating an envelope of continuous insulation which minimises heat loss. It has a design load of 130 kN/m2, making it highly durable, with excellent moisture-resistance and compressive strength. This enables the insulation to perform outside the waterproofing envelope. Installing insulation below the slab helps to avoid thermal bridges at floor and wall junctions and makes the most of precious internal space, meaning it is fast becoming recognised as an effective way of insulating new buildings.

Dwelling forms have been kept deliberately simple at the Hastoe development to avoid thermal bridging risks. Porches, meter boxes and Brise Soleil are all independently supported to avoid penetrating the insulation overcoat. East-West orientation of the units facilitates passive solar gain, with careful attention to shading to avoid summer overheating. The design and construction methods also assisted with the demanding air tightness requirement of 0.6 air changes per hour, with internal wet plaster providing the majority of the barrier and all joins covered in specialist membranes or tapes.

The elemental thermal values for the project:

External walls 0.088 W/m2/k

Floor 0.07 W/m2/k

Windows 0.79 W/m2/k

MVHR efficiency: 92%

Air changes per hour: 0.6 [n50]

About Thomas Armstrong

The Thomas Armstrong Group, the largest block manufacturer in the UK operates eight factories across northern England and into Scotland from its base in Cumbria. Founded in 1830, the group launched itself into aggregate blocks some 66 years ago and has, says company secretary and financial director Paul Armstrong, been investing heavily in increasing its capacity since the recession eased its grip in 2012. It is planning a further £10m investment in 2017.

In recent months, the group’s Stocks Bros subsidiary has launched a new £15m factory at Cross Green in Leeds where it now operates 15-hour days to meet demand. The factory is pressing more than 130,000 blocks a day — enough to build 60 to 70 average size homes. Overall, the group supplies sufficient for up to 200 homes per day.

“We have seen good demand since the world returned to normal in 2013-14,” says Paul. “The house builders are out there and we are doing all we can to satisfy demand. The problem lies in the need for more affordable housing but the solution to that is down to Government policy and not to product manufacturers.”

He adds: “If people want to invest £300,000 or £400,000 in a property they are going to be more receptive to buying a traditional brick and block home rather than a prefab.”

Paul sees sustainability as one of the block industry’s big advantages. His company aims to lead the way and plans to further strengthen its own case through the increasing use of carbon-neutral aggregate made from household incinerator ash waste. “We think that will stand up well against timber frame or any other type of construction,”

Andrew Minson of the Modern Masonry Alliance points out that the needed increase in block manufacturing is already in hand. The situation will be further aided by the measures proposed in the Housing White paper designed to encourage small and medium-sized contractors back into house building. The size of projects undertaken as a result is most likely to favour flexible off-the-shelf masonry solutions.

“Offsite construction has hitherto been limited in its application,” he adds. “It needs high throughputs and significant repetition to be cost effective. It also requires long term certainty to attract investors for the appropriate investment in time, essential research, production facilities and trained operatives. Whilst it may have a role to play, masonry was and will always remain the main solution.”